Air pollution Measurements and Emission Estimates

Dr. Wesam Al Madhoun

Types of measurements

1) Ambient measurement, Ambient Monitoring

2) Source measurement, source testing

The principal Basis of Air pollution law in US Which depends on Air Quality standard philosophy is to have Air safe to breath

This means that air quality should meet the NAAQS.

To achieve that: Control and Monitoring of Air pollutants should be implemented.

To control pollutant concentrations:

The time, the place, and the amount of emissions, should be regulated.

The emission rates of various sources of air pollutants must be regulated and measured.

Why measurements are needed?

1) Legal Requirements

2) Evaluation of air pollution control devices

Note: The control devices should be bought with Guarantee, and final payments should be after, demonstration that the device is installed ,calibrated and in good performance situation.

The Aim of Sampling:

•The principal requirement of a sampling system is to obtain a sample that is representative of the atmosphere at a particular place and time and that can be evaluated as a mass or volume concentration.

•The sampling system should not alter the chemical or physical characteristics of the sample in an undesirable manner.

The major components of most sampling

systems are:

1) An inlet manifold

2) An Air mover

3) A collection medium

4 ) Flow measurement device

(1) The inlet manifold transports the material from the

ambient atmosphere to the collection medium or

analytical device in an unaltered condition, all inlet of

ambient air must be rainproof.

(2) The air mover provides the force to create a

vacuum or lower pressure at the end of the sampling

system (pumps).

(3) The collecting medium, may be solid or liquid

sorbent for dissolving gases a filter surface for

collecting particles.

(4)The flow device measures the volume of air

associated with the sampling system.

A Representative sample

In Air measurements there are two important problems

1)To obtain the suitable representative sample

2)The determination of the concentration of the pollutant of interest in it correctly.

The representative ambient air sample has been the topic of legal and technical controversy.

(2) If the sample was taken from inside parking

structure, the result will show great violation of

NAAQS, specially CO.

(3) If the sample is taken directly across the street

from such a structure, in most cases the

concentration will be of magnitude less than inside

the structure.

(4) A block away, the concentration will be even

less.

(5) On the side walk directly adjacent to the

structure, the concentration will be may twice as

high as on the opposite side of the street.

•Which of these locations is suitable for obtaining a sample of ambient air?

•Generally the ambient air sampler should be located at the place to which the public has free access where the pollutant concentration is highest.

•This means that all indoor spaces, which the public has no access are excluded.

Where we can put an ambient air monitor ?

In Place:

1- has the power

2- shelter from rain and snow

3- constant temp. and pressure

4- easy access from monitoring personnel

5- protection from vandalism

6- if possible free rent place

•In the previous case, the suitable place is the

roof of county health building or of the county

courthouse.

•However the concentration of the outer-

related pollutants measured there is much

lower than at street levels at the busiest

intersection downtown.

•There are EPA guidelines for the proper placement

of intake of air samples that are to represent

ambient air, i.e. CO measurements must be made

at street level, downtown.

In source testing: The representative sample problem

is equally difficult

1- If the gas flow in large industrial smokestack is

steady, well mixed across the diameter of the stack. It

can be taken a representative sample any time and at

any place.

2- Most of such stacks have variation in velocities and

concentrations from point to point, and from time to

time.

In this case, many separate measurements Must be made and averaged.

In newer plants, designers sometimes

considered the problems of obtaining a

uniform gas flow and have provided

access and a suitable location for the

source sampler to place the required

instruments.

Getting the representative sample to the detector

Many sampling instruments have some kind of devices on the inlet to exclude unwanted materials. i.e.:

-Insects sucked into a particulate sampler nozzle lead to erroneously high readings.

-Dust particles of less health concern that weigh more than all of the fine particles.

Consistency between inlet excluding materials and modification of analysis.

EPA modified its particulate samplers in 1987 and in 1997.

•Before 1987, the sampler inlet was designed to exclude all particles larger than 50 micron, the quantity sampled was called Total Suspended particulate ( TSP).

•The 1987 modification changed the inlet to exclude all particles larger than 10 micron, the quantity sampled is called PM10.

•The 1997 modification changed the inlet again to exclude all particles larger than 2.5 micron. (PM2.5 or smaller), and also changed the flow rate.

•In sampling device, gases may condense in the sampling device, or react with the solids encounter there.

•Many combustion stack gases have a high water content and will condense on the walls of unheated sampling probe. In such cases, probes are normally heated to prevent this.

-If a grab sample is to be brought to the lab for

analysis, the sample container should be from

appropriate material and well pretreated according to

the analyte type .

i.e. glass containers can react with some air

pollutants.

Concentration Determination

•After sampling representatively and correctly,

determination of analyte concentration is followed.

•For some pollutants, can be measured easily by real

time instruments.

•Most of these operate optically, due to absorbance

and transmittance at a particulate wavelength. i.e.

Spectrophotometric methods.

The problem of interferences

•The interferences problem in air pollutants measurements is not trivial.

•Even in the spectrophotometric methods, some interferences gases can be absorbed on the same wavelength.

i.e. Measuring SO2 gas in N2 gas is easy, due to the reaction

SO2 + 2NaOH → Na2SO3 + H2O

And measuring the change in NaOH concentration

by simple acid base titration.

• However, if the problem is to measure SO2 in air,

CO2 in the air will also react as the following

reaction:

CO2 + 2NaOH → Na2CO3 + H2O

•If CO2 concentration in air in this case is steady and

known, so a correction could be made in principle

(but not in practice).

• But in the other cases, the interfering of the other

components may be not known, i.e. the US EPA

adopted a new method for measuring NO2 and had to

withdraw it, when became clear that the method was

not adequately protected from such interferences.

Averaging

• Measuring ambient air quality with real time

instruments, generally the average concentration

over some period of time is given, i.e.

Average concentration = Cavg= 1/∆t ∫c.dt

C= the instantaneous concentration

t= the time of measurement

•Most of real-time instruments present their results

as an electronic signal that can be easily averaged

by built-in electronics for any suitable chosen

averaging time.

•The older instruments for gases as well as the

current instruments, for particulate not real time

instruments, but rather are averaging instruments.

For example:

In EPA method for PM2.5 sampling, it consists of:

- Special inlet that exclude particles >2.5 micron in diameter

-A filter

-A fan

-A flow measuring device

-Suitable housing

Collection is for 24 hrs

Average Concentration = increase in filter weight / air flow rate x ∆t

Example 4.2:

In the example 4.2:

-The sample size is small

-The average value 21 µg/m3 is 84% of the annual avg NAAQS.

-Measured change in filter weight is 0.5 mg.

-High quality weighing and sample humidity control is needed

-The resulting statistical errors percentages,

can be high.

- The resulting measurements are upon 24

hours period, which means that this type of

instruments is not suitable for determining

hourly variations or trends as the real time

instruments.

Older measuring schemes for gas pollutants

have the principles of:

1- A measured volume of gas passing through

a bubbler contained solution that react

specifically (no interference) with the gas to be

measured.

2- Titration of the solution to determine the

concentration of the gas

N1 V1 = N2 V2

3- Or measuring the color of the solution, spectrophotometrically.

•Many source devices use the cumulative measuring schemes.

For example:

The EPA recommended sampling train for measuring the concentration of SO2 gas in a stack.

Standard Analytical Methods

EPA has standard methods for various pollutants. (often one for source and other for ambient air)

These methods define the pollutant. i.e.

SO2 is defined as the material which is detected

by the SO2 method shown in

EPA STTANDARD METHODS FOR MAJOR AIR

POLLUTANTS IN AMBIENT AIR.

However, in powerplant stack SO2 is defined as

that material that is detected by (Method 6, Fig.

4.5) which is chemically quite different from the

West Gaeke method.

•Test methods for major air pollutants in ambient air

Reference methods

Equivalent methods

•TSP, PM10, PM2.5

3 standard methods, differ in the excluding in the inlet .

Collecting on a filter for 24 hours

Averaging the results (weight difference)

SO2: West-Gaeke Method

•Known volume of air bubbled through a

solution of sodium tetrachloro mercurate

(TCM), which form a complex with SO2.

• After several reactions, solution treated with

pararosaniline (magenta dye, C19H18N3Cl), to

form intensity colored pararosaniline methyl

sulfonic acid.

•The concentration is determined colorimetric,

at wavelength 548 nm.

Ozone (chemioluminscence by etheylene)

•The air is mixed with ethylene, which reacts with ozone in a light-emitting reaction (chemioluminscence reaction).

• Chemiluminescence is the emission of light with limited emission of heat (luminescence), as the result of a chemical reaction.

•The light is measured with a photomultiplier tube related to concentration of the Ozone reacted.

CO

•It is measured by non-dispersive infrared absorption.

•Non-dispersive mean that the IR is not dispersed by a prism.

•Absorption of IR radiation by CO

•Detection due to energy difference signal

Note:

•This method is sensitive for water vapor interferences.

Hydrocarbons (non methane)

•Gas is passed through FID, where HC is burned in a hydrogen flame.

•HC cause more ionization than H2 and this ionization is detected electronically.

•Part of the sample is diverted for determination of direct CH4 by GC.

Note:

•HC determination is important even it is not NAAQS, for ozone control program.

NO2

• NO2 is converted to NO which is then reacted

with O3

•The light from this chemioluminscence reaction

is measured.

• Ambient air contains NO (often more than NO2),

so a parallel sample is run without convert NO2 to

NO.

• The resulting NO reading is subtracted from the

combined NO and NO2.

Lead

• A TSP filter extracted with HNO3 and HCL to dissolve the

lead.

• Atomic absorption spectroscopy (AAS) used to

determine the amount of lead in the extract.

• AAS: is a spectro-analytical procedure for the

qualitative and quantitative determination of chemical

elements employing the absorption of optical radiation

(light) by free atoms in the gaseous state.

Isokinetic Sampling

• In stack sampling for particulates, one must

maintain isokinetic flow into the sampling

probe.

Emission Factors

• Emission testing is expensive

•For simple, well-defined sources (power plant

stack), it can be done.

• For poorly defined sources (e.g, dust from

unpaved roads or Co from forest fires), reliable

test results are difficult to get.

• Testing only possible after the facility in Place.